Sample file attached. Created in RPR v2.0.52 with Blender 2.8 nightly.
My previous bug about multi coloured Hair using the ColorRamp was missing a vital node which I didn't know know about at the time.
The node is called "hair info" and it allows the Hair Intercept to be used as a Factor value on the ColorRamp.
This then allows the creation of muti-colour hair strands with normal shaders. e.g. to make the root of the hair darker than the tip.
RPR v2.0.52 Result for final render (preview just crashes):
Sample file attached.